Abstract:

A process for manufacturing a platinum resistance thermometer that is
stable with reduced resistance value variation over service temperature
range through clarification of a quantitative mechanism of resistance
value variation with respect to the oxidation/reduction of platinum
resistance wire. The process including the steps of (S1) enclosing a
purge gas containing an inert gas and oxygen in a protection tube
provided with a thermosensitive part of platinum resistance wire; (S2)
raising the internal temperature of the protection tube to a temperature
region in which the platinum is in reduced form at a partial pressure of
oxygen in the purge gas as determined from platinum oxide formation free
energy; (S3) replacing the purge gas with an inert gas wherein oxygen is
1 kPa or below; and (S4) sealing the protection tube under the replaced
condition.

Claims:

1. A process for manufacturing a platinum resistance thermometer provided
with a thermosensitive part composed of platinum resistance wire or
platinum resistance film in a protective tube, comprising the steps
of:enclosing a purge gas containing a first inert gas and oxygen in the
protective tube provided with the thermosensitive part composed of
platinum resistance wire or platinum resistance film;raising the
temperature inside the protective tube to a temperature region in which
the platinum is in reduced form at a partial pressure of oxygen in said
purge gas as determined from platinum oxide formation free energy;
andsubstituting said purge gas with a second inert gas in which oxygen is
1 kPa or less.

2. The process for manufacturing a platinum resistance thermometer
according to claim 1, further comprising the steps of sealing said
protective tube after said second inert gas has been substituted, such
that the thermometer can be stably used to the temperature region at
which the platinum is in reduced form at a partial pressure said oxygen
at 1 kPa or less as determined from platinum oxide formation free energy.

3. The process for manufacturing a platinum resistance thermometer
according to claim 1, further comprising the steps of taking a gas
containing oxygen as said purge gas and causing sufficient PtO2
oxidation of the surface of said platinum resistance wire or platinum
resistance film; and sealing said protective tube with the partial
pressure of the oxygen in said purge gas adjusted to a partial pressure
such that the use temperature region of the applicable platinum
resistance thermometer falls within the temperature region of said
oxidized form of PtO2, whereby the thermometer can be stably used in
the temperature region of the applicable oxidized form by lowering the
internal temperature of the protective tube, with the purge gas having
been substituted with said second inert gas, to the temperature region of
the oxidized form of PtO2, in which the partial pressure of the
oxygen in said purge gas is 1 kPa or less as determined from the platinum
oxide formation free energy.

4. The process for manufacturing a platinum resistance thermometer
according to claim 3 in which said purge gas containing oxygen, is a gas
of 100% oxygen in order to cause PtO2 oxidation the surface of said
platinum resistance wire or platinum resistance film.

5. The process for manufacturing a platinum resistance thermometer
according to claim 1 wherein said temperature region of platinum in
reduced form and said temperature region in PtO2 form are determined
as a function of respective oxide formation free energies.

6. The process for manufacturing a platinum resistance thermometer
according to claim 1 wherein said first inert gas with oxygen of 1 kPa or
less that is substituted is an inert gas with roughly 0% or trace amounts
of oxygen.

7. The process for manufacturing a platinum resistance thermometer
according to claim 6, wherein a substance obtained by passing a specified
highly pure inert gas through an oxygen getter is used as said first
inert gas in which oxygen is roughly 0%.

8. A platinum resistance thermometer manufactured by of the manufacturing
process according to claim 1.

9. The process for manufacturing a platinum resistance thermometer
according to claim 4, further comprising the steps of: sealing said
protective tube after said second inert gas has been substituted, such
that the thermometer can be stably used to the temperature region at
which the platinum is in reduced form at a partial pressure said oxygen
at 1 kPa or less as determined from platinum oxide formation free energy.

10. The process for manufacturing a platinum resistance thermometer
according to claim 5, further comprising the steps of: sealing said
protective tube after said second inert gas has been substituted, such
that the thermometer can be stably used to the temperature region at
which the platinum is in reduced form at a partial pressure said oxygen
at 1 kPa or less as determined from platinum oxide formation free energy.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001]This is the U.S. national phase application under 35 U.S.C.
§371 of International Patent Application No. PCT/JP2008/059156 filed
May 19, 2008 and claims the benefit of Japanese Application No.
2007-133383, filed May 18, 2007. The International Application was
published on Nov. 27, 2008 as International Publication No.
WO/2008/143221 under PCT Article 21(2) the contents of these applications
are incorporated herein in their entirety.

FIELD OF TECHNOLOGY

[0002]The present invention relates to a manufacturing process in which a
platinum resistance thermometer can be configured to have less resistance
value variation and extremely stable characteristics by optimization of
the oxygen concentration in purge gas within the protective tube (sheath)
and of the temperature of such processes as annealing.

BACKGROUND OF THE INVENTION

[0003]Standard platinum resistance thermometers have been used for 50
years as secondary thermometers for determining temperature from standard
resistance values in research on temperature standards and in the
International Temperature Scale (ITS-90) that require reproducibility and
precision of under 0.001° C. FIG. 13 indicates the basic structure
of a conventional standard platinum resistance thermometer 1 described in
Japan Unexamined Patent Publication No. 2001-343291. Normally, in order
to prevent contamination of a highly pure platinum wire 20 that composes
a thermosensitive part 2, a gas impermeable protective tube 3 made of
quartz, sapphire or the like is used, and purging inside this protective
tube 3 is conducted with a mixed gas to which oxygen is intentionally
added, for example, argon 85%-oxygen 15%. The small amount of oxygen in
this purge gas G is present in order to protect the platinum wire 20 of
the thermosensitive part 2 from contamination, and there is no clear
standard for the amount, which is determined based on past experience and
varies for each manufacturer.

[0004]Nonetheless, by adding this oxygen, oxidation of the platinum
resistance wire by the oxygen in the purge gas causes the resistance
value at temperatures of 300° C. to 500° C. to gradually
increase, and variations equivalent to 0.001° C. (1 mK) or more
occur. Moreover, when exceeding 600° C., the resistance value
variations caused by this oxidation is reversed and the resistance value
returns to prior to oxidation. Consequently, if a standard platinum
resistance thermometer is used to straddle these two temperature regions,
the resistance value of the platinum wire gradually increases and is
unstable in the temperature region of 300° C. to 500° C.,
and then the increased portion of the resistance value is eliminated in
the temperature region of 600° C. or more, resulting in the
problem that there is no reproducibility in the relationship between the
resistance value and the temperature when used again in the temperature
region of 300° C. to 500° C.

[0005]As a conventional countermeasure, rather than use a standard
platinum resistance thermometer in the aforementioned two regions, use
was divided between platinum resistance thermometers specific to each
temperature region, and maintenance was conducted in accordance to the
use conditions (re-annealing and the like) for each thermometer while
confirming the characteristics (resistance value drift). These
countermeasures were taken because the mechanisms of quantitative
resistance value variation by oxidation-reduction of the platinum
resistance wire were not understood, and there was a lack of knowledge
about changes of platinum resistance wire caused by the oxygen
concentration in the purge gas when manufacturing standard platinum
resistance thermometers. The above problems are not limited to standard
platinum resistance thermometers, but are the same for quasi-standard and
industrial ones as well, and the uncertainty caused by oxidation of the
platinum has not been discussed in the field of temperature measurements
using platinum resistance thermometers and has been virtually ignored in
the ITS-90 interpolation formulae in the field of platinum resistance
thermometers. The affects of oxidation resistance values cannot be
ignored in precision temperature measurements.

SUMMARY OF THE INVENTION

[0006]In view of the previously described circumstances, in an attempt at
resolution, the point of the present invention is to demonstrate the
mechanisms of quantitative resistance value variation by
oxidation-reduction of the platinum resistance wire, and to offer a
manufacturing process to obtain a stable platinum resistance thermometer
with less resistance value variation in the temperature region of use.

[0007]As a result of assiduous studies related to the present invention in
order to resolve the previously described problems, knowledge was
obtained regarding the relationship between the variations of platinum
resistance thermometer resistance values and oxidation potential phase
charts, and the following mechanism regarding quantitative resistance
value variation by oxidation-reduction of platinum resistance thermometer
resistance wire was demonstrated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is an oxidation potential phase chart in which the platinum
oxide formation free energies at partial pressures of 0.1 kPa and 10 kPa
of oxygen have been calculated;

[0009]FIG. 2 is a graph of measurements of the amount of resistance value
drift of platinum resistance thermometers with resistance caused by
oxidation-reduction at a given temperature;

[0010]FIG. 3 is a graph indicating the amount of initial drift variation
caused by oxidation-reduction at 200° C. to 570° C.;

[0011]FIG. 4 is a graph indicating the results of measuring the resistance
values while maintaining Example 1 and Comparative Example 1 at
420° C. for 100 hours or more, and at 400° C. for 100 hours
or more;

[0012]FIG. 5 is a graph indicating the results of measuring the resistance
values after using Example 1 and Comparative Example 1 for 1000 hours or
more in the temperature region of 230 to 420° C., and then causing
isothermal reduction at 480° C. and 510° C.;

[0013]FIG. 6 is a graph indicating the results of measuring resistance
value variations of Embodiments 2, 3 and Comparative Examples 2 to 4 at
230° C.;

[0015]FIG. 8 is a graph indicating the results of measuring resistance
value variations of platinum resistance thermometers at 100° C.;

[0016]FIG. 9 is a graph indicating the results of measuring resistance
value variations of platinum resistance thermometers at 150° C.;

[0017]FIG. 10 is a graph indicating the results of maintaining platinum
resistance thermometers at 420° C. for approximately 15 hours,
afterwards cooling in order to cause full oxidation of the platinum wire,
and then measuring resistance values;

[0018]FIG. 11 is a flowchart indicating the order of manufacturing the
first embodiment;

[0019]FIG. 12 is a flowchart indicating the order of manufacturing the
second embodiment; and

[0022]FIG. 1 is an oxidation potential phase chart relating to the
oxidation of platinum, which is calculated by the oxide formation free
energy (Gibbs free energy) of the platinum oxides PtO2 and PtO in
oxidation reactions when the oxygen partial pressures were 0.1 kPa and 10
kPa. These calculations were made using the Chemical Reaction and
Equilibrium Software with Extensive Thermochemical Database, Outokumpu
HSC Chemistry for Window, Ver. 5.0. Platinum oxides also include
Pt3O4 and the like, but of this type of platinum oxide may be
omitted because the oxide formation free energy is sufficiently larger
than that of PtO2 and PtO and has no direct affect on the various of
platinum resistance thermometer resistance values. The oxidation
potential phase chart of FIG. 1 indicates an energy balance wherein
PtO2 is formed in the equilibrium state at a temperature in which
the platinum in the oxidation environment is in the vicinity of
300° C., and PtO is formed at a temperature higher than that.
These chemical phase transitions demonstrate that the resistance of
platinum wire changes at these temperature vicinities. If this oxide film
is limited to the surface of the platinum wire, it may be inferred that
the resistance change is smaller than the change of resistance value in
the equilibrium state.

[0023]In the past it was said that the oxygen in the purge gas that is
sealed in the sheath of the platinum resistance thermometer is necessary
to protect the platinum wire from contamination by other metallic
impurities, and oxygen with a partial pressure of about 5 kPa at room
temperature was put into the platinum resistance thermometer sheath.
According to the oxidation potential phase chart in FIG. 1, PtO2 in
the equilibrium state at room temperature changes to PtO at 300°
C. to 450° C., and is reduced to platinum and oxygen at
temperatures above that. The characteristics of the equilibrium state of
the aggregate of platinum and oxygen are determined by the oxide
formation free energy, and the oxide formation free energy is determined
by the temperature t and the oxygen partial pressure p. The partial
pressure of oxygen within the platinum resistance thermometer is normally
adjusted to approximately 10 kPa or less, and the oxidation
characteristics of most platinum resistance thermometers are between the
two partial pressure lines in this chart.

[0024]The PtO2 chemical reaction is indicated in equation (1) below.
Moreover, the oxide formation free energy ΔG.sub.PtO2(T,p) of this
reaction is expressed by equation (2) below. Here, p is the oxygen
partial pressure; K.sub.PtO2(T) is the chemical equilibrium constant at
temperature T; and R is the gas constant. This equation indicates that
PtO2 is stable in the temperature region of
ΔG.sub.PtO2(T,p)<0, and that as long as oxygen is supplied, the
chemical reaction always advances to the right in reaction equation (1).
Moreover, at approximately 400° C. or more, there is another phase
transition, and the chemical reaction of PtO is indicated in equation (3)
below. The oxide formation free energy ΔG.sub.PtO(T,p) of this
reaction is expressed by equation (4) below. This reaction also depends
on the partial pressure p and the temperature T. The direction of this
reaction is also determined by the the sign of ΔG.sub.PtO(T,p). At
approximately 500° C. or more, ΔG.sub.PtO(T,p) becomes
>0, and PTO decomposes into Pt and O2.

Pt+O2=PtO2 (1)

ΔG.sub.PtO2(T,p)=-RT ln(K.sub.PtO2(T)/p) (2)

Pt+1/2O2=PtO (3)

ΔG.sub.PtO(T,p)=-RT ln(K.sub.PtO(T)/p1/2) (4)

[0025]Moreover, at 300° C. or less, the above reactions are
determined by the sizes of ΔG.sub.PtO2(T,p) and
ΔG.sub.PtO(T,p), and if ΔG.sub.PtO2(T,p) is smaller than
ΔG.sub.PtO(T,p), then PtO2 is stable. Therefore, PtO is stable
in the narrow temperature region in the vicinity of 400° C.
Specifically, only in the temperature region from the part that crosses
the PtO2 line in FIG. 1 to ΔG.sub.PtO(T,p)=0. In the
equilibrium state, enclosing approximately 10 kPa of oxygen gas should
contribute to a resistance value increase of about 1/10K, but because the
velocity of equilibration is extremely slow at room temperature and it
may be inferred that an oxide film on the surface of the platinum wire
limits diffusion of oxidation to the interior, the actual increase in
resistance value does not reach the resistance value in the equilibrium
state. Nonetheless, the platinum oxide increases over time and is
measured as resistance value drift. Moreover, this oxide reaction
velocity becomes faster at high temperatures. Therefore, resistance value
drift causes inaccuracy when taking precise measurements, specifically,
the measurements must be considered inaccurate at 300° C. or more.

[0026]The oxide formation free energy indicated in FIG. 1 indicates that
the oxidation-reduction reactions of platinum are controlled depending on
the oxygen partial pressure and the temperature. Phase change occurs at
the point where the 2 curves of a given partial pressure cross, and if
the platinum resistance thermometer is in the equilibrium state,
resistance change may be inferred based on these curves. The present
inventors conducted experiments to investigate the characteristics
(resistance value changes) of oxidation-reduction of platinum wire caused
by oxygen in the purge gas, and confirmed by actual measurements when not
in the equilibrium state that oxidation-reduction reactions are generated
following the oxide formation free energy lines in the oxidation
potential phase chart of FIG. 1, and that the resistance value of the
platinum resistance thermometer changes.

[0027]The platinum resistance thermometer actually used in the experiment
was the same as the conventional platinum resistance thermometer
indicated in FIG. 13, which had a structure in which platinum wire was
wound in a single coil shape on a quartz reel in a quartz sheath, and
which was modified so that the oxygen partial pressure in the sheath
could be adjusted. Three platinum resistance thermometers were used in
the experiment, and these were maintained at a temperature of 600°
C. or more for approximately 10 hours to cause reduction of the platinum
wire in the thermosensitive part. The partial pressures of the oxygen in
the purge gas of the respective platinum resistance thermometers were set
at roughly 2 kPa (Y002), 2 kPa (Y003), and roughly 8 kPa (S4742); these
were heated from 16 to 24 hours at suitable temperatures from 200°
C. to 500° C., and 600° C.; and the resistance value at the
triple point of water was measured every 8 hours.

[0028]FIG. 2 is a graph of measurements of the amount of drift of the
resistance values (not the heat equilibrium state) of platinum resistance
thermometers caused by oxidation-reduction at a given temperature for
each platinum resistance thermometer, and the transverse axis indicates
the exposure temperature while the longitudinal axis is the
temperature-converted value of the change of resistance value from when
reduced. It was demonstrated that the resistance value separated into 2
phases at a temperature of approximately 350° C. to 400°
C., and that the phase transitioned from PtO2 to PtO. Specifically,
when an oxidation potential phase chart like that of FIG. 1 is plotted,
the oxygen partial pressures of the various platinum resistance
thermometers at the two oxidation phases of platinum correspond to both
sides of the intersection point of the two intersecting curves. Moreover,
it was demonstrated that the resistance value decreased at approximately
450° C. to 530° C., and the phase transitioned from PtO to
Pt. When an oxidation potential phase chart like that of FIG. 1 is
plotted in the same way, the oxygen partial pressures of the various
platinum resistance thermometers correspond to both sides of the
intersection point of the PtO curve and the energy 0 line. According to
FIG. 2, there two steps in the resistance change of the various platinum
resistance thermometers, and the characteristic difference between these
platinum resistance thermometers is mainly the oxygen partial pressure.

[0029]These experiments confirm that even in the actual measurement
environment, which is not an equilibrium state, platinum reacts with
oxygen in accordance with the oxidation potential phase chart in FIG. 1.
Specifically, it was confirmed that the reaction from platinum to
PtO2, from PtO2 to PtO, and from PtO to Pt occur following the
oxide formation free energy lines of FIG. 1, and the characteristics of
resistance value variations are affected by the oxygen partial pressure
in the sheath. The reasons that the resistance value increases appear to
be that the number of conduction electrons of the platinum is decreased
by PtO2 chemically changing to 2PtO, and that the temperature
increase causes acceleration of the chemical change.

[0030]FIG. 3 indicates the change of initial drift caused by the
oxidation-reduction from 200° C. to 570° C. that was
experimentally obtained, and the longitudinal axis indicates the standard
uncertainty of the drift rate at specific temperatures. The drift rate is
slow in the PtO2 region and is fast in the PtO region. Therefore it
appears that the oxidation in the PtO temperature region is faster than
in the PtO2 temperature region. Moreover, the chart indicates that
oxidation is already progressing even at 200° C. or less.

[0031]Based on the findings that the oxygen in the purge gas and the
platinum wire in the platinum resistance thermometer react according to
the oxidation potential phase chart of platinum as described above and
that the characteristics of the resistance value variations are
determined by the partial pressure of the oxygen in the purge gas, the
present inventors discovered as a result of repeated experiments and
studies under various conditions that, by adjusting the oxygen in the
purge gas to a partial pressure of 1 kPa or less once having been
annealed by raising the internal temperature up to the reduction
temperature of platinum, a platinum resistance thermometer extremely
stable across the entire temperature region can be realized; and further
discovered that by causing sufficient oxidation of the surface of the
platinum by PtO2, if at a partial pressure at which the amount of
oxygen in the purge gas becomes the above PtO2 oxidized form in the
temperature region of use, a platinum resistance thermometer can be
realized, which can be used stably in the applicable temperature region
of use without necessarily restricting the partial pressure to 1 kPa.
Thus, the present invention was perfected.

[0032]Specifically, the present invention offers a process for
manufacturing a platinum resistance thermometer provided with a
thermosensitive part composed of platinum resistance wire or platinum
resistance film in a protective tube, wherein after enclosing a purge gas
containing an inert gas and oxygen in the protective tube provided with
the thermosensitive part composed of platinum resistance wire or platinum
resistance film and raising the temperature inside the protective tube to
temperature region in which the platinum is in reduced form at a partial
pressure of oxygen in the aforementioned purge gas as determined from
platinum oxide formation free energy, the aforementioned purge gas is
substituted with inert gas in which oxygen is 1 kPa or less. Further, in
the present application, the oxygen partial pressure of the purge gas is
the partial pressure at room temperature.

[0033]Here, preferably, by sealing the aforementioned protective tube in
the state with the aforementioned inert gas having been substituted, the
platinum resistance thermometer can be stably used to a temperature
region in which the platinum is in reduced form at a partial pressure of
oxygen in the purge gas at 1 kPa or less as determined from platinum
oxide formation free energy.

[0034]Or, preferably, with the purge gas having been substituted with the
aforementioned inert gas, after the internal temperature of the
protective tube has been lowered to the temperature region of the
oxidized form of PtO2, which is derived from the platinum oxide
formation free energy with the partial pressure of the oxygen in the
aforementioned purge gas at 1 kPa or less, by taking a gas containing
oxygen as the aforementioned purge gas, causing sufficient PtO2
oxidation of the surface of the aforementioned platinum resistance wire
or platinum resistance film, and sealing the aforementioned protective
tube with the partial pressure of the oxygen in the aforementioned purge
gas adjusted to a partial pressure such that the use temperature region
of the applicable platinum resistance thermometer falls within the
temperature region of the aforementioned oxidized form of PtO2 (a
temperature lower than the temperature at which PtO begins to form), the
platinum resistance thermometer can be stably used in the temperature
region of the applicable oxidized form.

[0035]Here, in order to cause PtO2 oxidation of the surface of the
aforementioned platinum resistance wire or platinum resistance film,
preferably, the applicable oxidized form is stabilized by causing
sufficient PtO2 oxidation the surface of the aforementioned platinum
resistance wire or platinum resistance film using gas with 100% oxygen as
the aforementioned gas containing oxygen.

[0036]Moreover, preferably, the temperature region of the aforementioned
reduced form of platinum and the temperature region of the aforementioned
PtO2 form are derived using the oxidation potential phase chart of
platinum based on the respective oxide formation free energies.

[0037]Further, preferably the aforementioned inert gas with oxygen of 1
kPa or less to be substituted is an inert gas containing roughly 0% or
trace amounts of oxygen.

[0038]More concretely, preferably the specified highly pure inert gas that
has been passed through an oxygen getter is used for the aforementioned
inert gas with roughly 0% oxygen.

[0040]Because purge gas containing inert gas and oxygen is enclosed in the
protective tube provided with a thermosensitive part of platinum
resistance wire or platinum resistance film, the temperature inside the
protective tube is raised to a temperature region in which the platinum
is in reduced form at a partial pressure of oxygen in the purge gas as
determined from platinum oxide formation free energy, and then the
aforementioned purge gas is replaced with inert gas in which oxygen is 1
kPa or less, the invention of the present application has stable
characteristics with less resistance variation irrespective of
temperature by increasing the purity (resistance ratio) of the platinum
resistance wire or the like, conducting residual strain relief annealing
of the platinum resistance wire in a clean high temperature atmosphere
containing oxygen, and, once a substrate with stable resistance value has
been made, by making the oxygen partial pressure of the purge gas be 1
kPa or less.

[0041]In particular, by sealing the protective tube when inert gas with 1
kPa or less of oxygen has been substituted, a highly reliable platinum
resistance thermometer can be offered with less resistance value
variation, satisfactory measurement reproducibility and stable
performance over a broad use temperature region comprising both the
oxidation and reduction regions, specifically from the low temperature
region to the temperature region (high temperature region) in which the
platinum is in reduced form at a partial pressure of oxygen of 1 kPa or
less in the applicable purge gas as determined by the platinum oxide
formation free energy.

[0042]Moreover, instead of sealing the protective tube when inert gas with
1 kPa or less of oxygen has been substituted as stated above, by reducing
the temperature inside the protective tube, which has purge gas
substituted by the aforementioned inert gas, to the temperature region of
the PtO2 oxidation form as determined by the platinum oxide
formation free energy of the applicable oxygen partial pressure, then by
causing sufficient PtO2 oxidation of the surface of the
aforementioned platinum resistance wire or platinum resistance film, and
by further sealing the aforementioned protective tube with the oxygen
partial pressure of the aforementioned purge gas adjusted to the partial
pressure such that the use temperature region of the applicable platinum
resistance thermometer falls within the temperature region of the
aforementioned oxidation form of PtO2, a platinum resistance
thermometer can be offered that has no PtO form change and that can be
stably used with less resistance value variation in the temperature
region of the applicable oxidized form.

[0043]The temperature region of the reduced form of platinum and the
temperature of the PtO2 form can be effectively determined using the
platinum oxidation potential phase chart based on the respective oxide
formation free energies.

[0044]More preferably, by taking an inert gas with roughly 0% or trace
amounts (partial pressure of about 10 Pa or less) of oxygen as the inert
gas to be substituted in which oxygen is 1 kPa or less, a platinum
resistance thermometer with a more stable resistance value can be offered
when sealing the protective tube with the applicable substitution gas. In
the above cases in which further oxidation is conducted prior to sealing,
the oxidation reaction speed is suppressed in the PtO region when
lowering the temperature to the oxidation temperature, and therefore the
production of PtO is suppressed and oxidation as pure PtO2 can be
conducted, which can stabilize the resistance value.

[0045]Moreover, inert gas with roughly 0% oxygen can be effectively
obtained by further passing highly pure inert gas containing 0.2 ppm to
several ppm of oxygen through an oxygen getter such as a sponge titanium.

BEST MODE FOR CARRYING OUT THE INVENTION

[0046]Next, embodiments of the present invention will be explained in
detail based on the attached drawings.

[0047]FIG. 11 is a step chart indicating the order of manufacturing
Example 1; FIG. 12 is a step chart indicating the order of manufacturing
Embodiment 2; and FIG. 13 indicates the structure of representative
platinum resistance thermometer as used in the past. Further, in the
embodiments below, an explanation is given of an example of a platinum
resistance thermometer with a structure comprising a thermosensitive part
with a single coil winding formed in a coil shape by winding straight
platinum resistance wire 20 on cross-shaped reel 4 as indicated in FIG.
13, but the structure of the platinum resistance thermometer of the
present invention is not limited to this, and a variety of structures
well-known in the past may be adopted, for example, a structure in which
coil-shaped platinum wire inserted in 2 spiral-shaped quartz tubules, or
a structure that has a double coil type thermosensitive part in which
coil-shaped platinum resistance wire is wound in a coil shape in grooves
formed on a cross-shaped quartz reel.

[0048]Instead of providing platinum resistance wire, the thermosensitive
part may be configured by vapor deposition formation of a platinum
resistance film, or the like. As long as there is a structure in which
the thermosensitive part comprising platinum resistance wire or platinum
resistance film is provided in a protective tube and purge gas is sealed
in, any structure is acceptable. The materials used in the past for the
materials of the configurational members (protective tube and the like)
may be broadly adopted, and are not particularly limited in regard to use
such as for standard, quasi-standard, or industrial use. The inert gas
used in the purge gas is not particularly limited, and argon, nitrogen,
helium or neon can be suitably used. First, the process of manufacturing
the platinum resistance thermometer related to a first embodiment will be
explained based on FIG. 11 and FIG. 13.

[0049]The process for manufacturing the present embodiment at least
comprises a gas enclosing step Si of enclosing a purge gas G containing
inert gas and oxygen in a protective tube 3 provided with a
thermosensitive part 2 composed of a platinum resistance wire 20; a
temperature-raising step S2 of raising the internal temperature of the
protective tube 3 to the temperature region in which the platinum is in
reduced form at a partial pressure of the oxygen in the aforementioned
purge gas G as determined by the platinum oxide formation free energy; a
gas substitution step 3 of substituting the aforementioned purge gas G
with inert gas in which oxygen is 1 kPa or less; and a sealing step 4 of
sealing the protective tube 3 in this substituted state. The present
embodiment is thereby capable of being stably used in the temperature
region in which the platinum is in reduced form at a oxygen partial
pressure of 1 kPa or less as determined by the platinum oxide formation
free energy.

[0050]A suitable amount of oxygen is included in the purge gas from the
gas enclosing step S1 to the temperature-raising step S2 in order to
oxidize and remove impurities adhering to the various parts in the
protective tube, and to the surface of the platinum resistance wire in
particular, and after the temperature-raising step S2 is complete the
oxygen concentration is adjusted by substituting with the aforementioned
inert gas in which the oxygen partial pressure is 1 kPa or less. In the
temperature-raising step S2, the temperature is raised to the Pt
reduction temperature region based on the previously described oxygen
partial pressure of the purge gas that has been enclosed and on the
aforementioned platinum oxide formation free energy as determined by the
platinum potential phase chart. As the aforementioned impurities are
removed under a clean high temperature atmosphere containing oxygen,
oxygen is removed and reduced from the oxides on the surface of the
platinum resistance wire, which is turned into clear platinum wire. At
the same time, the resistance ratio of the platinum resistance wire is
raised based on the effect as residual strain relief annealing of the
platinum resistance wire, which is conducted in order to increase the
so-called "purity" of the platinum wire. The steps from the gas enclosing
step S1 to the temperature-raising step S2 may be repeated multiple
times. Moreover, a more preferable embodiment conducts pre-processing by
repeatedly conducting annealing and gas substitution at a lower
temperature at a stage prior to the gas enclosing step S1 for the
temperature-raising step S2.

[0051]Then, after the temperature-raising step S2 is completed,
substitution with the substitution gas for final enclosure is conducted
at the gas substitution step S3, and in the present invention, by keeping
the oxygen partial pressure of this substitution gas to 1 kPa or less, a
platinum resistance thermometer can be obtained with less resistance
value variation and high reproducibility even when used in both the
oxidation and reduction temperature ranges. The oxygen partial pressure
is preferably set to a trace amount, concretely, to 10 Pa or less, more
preferably to 1 Pa or less, and even more preferably to 0.1 Pa or less;
most desirably, the oxygen partial pressure is set to an oxygen
concentration of roughly 0% obtained by passing the specified highly pure
inert gas though an oxygen getter.

[0052]Next, the process for manufacturing a platinum resistance
thermometer related to a second embodiment will be explained based on
FIG. 12 and FIG. 13.

[0053]The process for manufacturing the present embodiment at least
comprises a gas enclosing step S1 of enclosing a purge gas G containing
inert gas and oxygen in a protective tube 3 provided with a
thermosensitive part 2 composed of a platinum resistance wire 20; a
temperature-raising step S2 of raising the internal temperature to the
temperature region in which the Pt is in reduced form at a partial
pressure of the oxygen in the aforementioned purge gas G as determined by
the platinum oxide formation free energy; a gas substitution step 3 of
substituting the aforementioned purge gas G with inert gas in which
oxygen is 1 kPa or less; a temperature lowering step S4 of lowering the
internal temperature of the protective tube with the substituted purge
gas G to the temperature region of the oxidized form of PtO2 in
which the partial pressure of the oxygen in the aforementioned purge gas
G is 1 kPa or less as determined by the platinum oxide formation free
energy; an oxidation step S5 of causing PtO2 oxidation on the
surface of the platinum resistance wire 20 based on the purge gas G
containing oxygen; a gas adjusting step S6 of adjusting the oxygen
partial pressure of the purge gas G to a partial pressure such that the
use temperature region of the applicable platinum resistance thermometer
1 falls within the temperature region of aforementioned oxidized form of
PtO2; and a sealing step 7 of sealing the protective tube 3 in this
oxygen partial pressure adjusted state. In the present the embodiment, in
the same way as in the above first embodiment, an order was adopted in
which, after the purity is increased by raising the temperature to the
reduction region in a gas atmosphere containing oxygen, eliminating the
impurities adhering to the platinum wire, and reducing the PtO, etc. on
the surface and conducting residual strain relief annealing, the
temperature is decreased to the PtO2 [region] in a form with the
oxygen partial pressure set low in order as much as possible not to
produce PtO on the surface of the platinum wire in the PtO region,
through which the process passes when lowering the temperature to the
PtO2 region. Concretely, at the gas substitution step S3 after
raising the temperature, the purge gas is replaced with gas with a low
oxygen content in which the oxygen partial pressure is 1 kPa or less, and
the temperature is lowered in that state. With the oxygen partial
pressure as low as possible at this time, it is possible to pass rapidly
through the PtO region, and the oxygen partial pressure is preferably set
to a trace amount, concretely, to 10 Pa or less, more preferably to 1 Pa
or less, and even more preferably to 0.1 Pa or less; most desirably, the
oxygen partial pressure is set to an oxygen concentration of roughly 0%
obtained by passing the specified highly pure inert gas though an oxygen
getter.

[0054]In the temperature lowering step S4, the PtO2 region is
determined by the platinum oxidation potential phase chart based on the
oxygen partial pressure of the previously described low oxygen content
gas that was replaced and on the oxide formation free energy, and
preferably cooling is conducted such that the temperature lowering rate
is rapid allowing quicker passage through the PtO region. Then in the
oxidation step S5, with the temperature lowered to the PtO2 region,
high oxygen concentration purge gas is substituted and the surface of the
platinum wire is allowed to sufficiently oxidize, and preferably the gas
used at that time has an oxygen concentration of nearly 100%.

[0055]Then, in the final gas adjustment step S6 prior to sealing, as
opposed to the aforementioned first embodiment, the oxygen partial
pressure in the purge gas is not set to 1 kPa or less, but rather it is
necessary to set the oxygen partial pressure of the purge gas to greater
than 1 kPa in order to make a partial pressure such that the use
temperature region falls within the aforementioned PtO2 region (does
not change form to PtO). For example, if making the use region a
temperature region from low temperature to near 300° C., the
oxygen partial pressure as determined from the platinum oxidation
potential phase chart indicated in FIG. 1 is adjusted to about 10 kPa or
more.

[0056]An embodiment of the present invention was described above, but the
present invention is not at all limited by this embodiment, and of course
a variety of forms that are within the range that does not deviate from
the intention of the present invention may be implemented.

Examples

(Experiment 1)

[0057]An experiment was conducted to investigate the oxidation-reduction
characteristics in the high temperature region using a platinum
resistance thermometer of Example 1 produced by the manufacturing process
of the first embodiment above and a platinum resistance thermometer of
Comparative Example 1, in which only the oxygen partial pressure was
modified. The oxygen partial pressure of Example 1 was a trace amount of
approximately 0.1 Pa, and the oxygen partial pressure of Comparative
Example 1 was 4 kPa at room temperature. The reduction and annealing
prior to gas substitution was conducted at a temperature of 670°
C. for 10 hours. In Example 1 highly pure argon (oxygen partial pressure
of approximately 0.1 Pa) was substituted as the purge gas, and
Comparative Example 1 was adjusted such that the oxygen partial pressure
was 4 kPa at room temperature, and such that together with argon the
total pressure was approximately 100 kPa or less at 900° C. and
approximately 25 kPa at room temperature.

[0058]First, the platinum resistance thermometers were maintained at
420° C. for 100 hours or more, and at 400° C. for 100 hours
or more, and periodically were cooled to room temperature to measure the
resistance values of temperatures at the triple point of water. The
measurement results are indicated in FIG. 4. In Comparative Example 1 (4
kPa), there was an increase of resistance value in conjunction with the
time of exposure at 420° C. and 400° C. As is evident from
the oxidation potential phase chart in FIG. 1, ΔG.sub.PtO(T,p) is
negative in this temperature region, and the O2 in the sheath is
consumed and PtO spreads into the platinum wire. Meanwhile, in Example 1
(approximately 0.1 Pa), the resistance values at 420° C. and
400° C. were within about 1 mK and were maintained at a constant
level. At the oxygen partial pressure of approximately 0.1 Pa, the
ΔG.sub.PtO and the ΔG.sub.PtO2 are both positive at
420° C. and 400° C., and the platinum wire does not
oxidize. The different results of these two platinum resistance
thermometers indicate that the oxygen partial pressure in the sheath
greatly contributes to the variations of resistance value, and
demonstrate that an oxygen partial pressure of approximately 0.1 Pa is
extremely stable in relation to oxidation, and is suitable for precision
measurements in the temperature region of 400° C. or more.

[0059]Next, after measuring the aforementioned resistance values of
oxidation characteristics, and after having used Example 1 (approximately
0.1 Pa) and Comparative Example 1 (4 kPa) for 1000 hours or more in the
230 to 420° C. temperature range, isothermic reduction was
measured at 480° C. and 510° C. The results of isothermic
reduction are indicated in FIG. 5. In Comparative Example 1, in the
oxidation potential phase chart the ΔG.sub.PtO(T) at 480° C.
nearly traverses the line ΔG.sub.PtO(T)=0, the resistance value is
nearly constant as indicated in FIG. 5, and PtO and Pt are in
equilibrium. However, at 510° C., ΔG.sub.PtO(T) becomes
>0, and it is clear that reduction of PtO causes a rapid decrease in
resistance value. Meanwhile, the resistance value of Example 1 is
constant at 480° C. and 510° C. The results of Experiment 1
above demonstrate that there are large resistance value variations caused
by oxidation and reduction in the platinum resistance thermometer in
which the oxygen partial pressure is 4 kPa, but the platinum resistance
thermometer with the low oxygen partial pressure of approximately 0.1 Pa
is stable at any temperature.

(Experiment 2)

[0060]Next, an experiment was conducted to investigate the isothermic
oxidation characteristics using example and comparative platinum
resistance thermometers manufactured following the above manufacturing
method of the first embodiment and set to multiple and more detailed
final adjustments of the oxygen partial pressures. Using 5 types of
platinum resistance thermometer with differing oxygen partial pressures,
Example 2 (approximately 0.1 Pa), Example 3 (0.8 kPa), Comparative
Example 2 (2 kPa), Comparative Example 3 (4 kPa), and comparative Example
4 (8 kPa), reduction and annealing prior to the respective gas
substitutions was conducted at a temperature of 670° C. for 10
hours. The numeric values in parentheses of the various platinum
resistance thermometers are the respective oxygen partial pressures at
room temperature. FIG. 6 indicates the results of measuring the
resistance value variations of the platinum resistance thermometers at
230° C., and FIG. 7 indicates the results of measuring the
resistance value variations of the platinum resistance thermometers
(except for Comparative Example 4) at 420° C.

[0061]As indicated in FIG. 6, in low oxygen partial pressures such as
Example 2 (approximately 0.1 Pa) and Example 3 (0.8 kPa) drift at
230° C. was comparatively small, but Comparative example 2 (2
kPa), Comparative Example 3 (4 kPa) and Comparative Example 4 (8 kPa)
were not stable even at the low temperature of 230° C.
Specifically, Comparative Example 3 (4 kPa) and Comparative Example 4 (8
kPa) exhibited large resistance variations of 60 mK or more. Meanwhile,
as indicated in FIG. 7, it was demonstrated that low oxygen partial
pressures such as Example 2 (approximately 0.1 Pa) and Example 3 (0.8
kPa) were stable even at 420° C. Comparative Example 2 (2 kPa) was
comparatively stable at 420° C., but resistance value drift was
observed at 230° C. This fact indicates that the oxygen partial
pressure is extremely sensitive to oxidation. This demonstrates that the
oxygen partial pressure must be adjusted according to the temperature
region to be used, and that a platinum resistance thermometer capable of
all temperature ranges must adjusted to a low oxygen partial pressure of
1 kPa or less.

[0062]As demonstrated by Experiments 1 and 2, inasmuch as resistance value
drift of platinum resistance thermometers depends on the chemical
reactions of PtO2 and PtO, which are controlled by the oxide
formation free energy, platinum resistance thermometers with a high
oxygen partial pressure indicate large resistance value variations when
used at 400° C. or more. Meanwhile, platinum resistance
thermometers with a low oxygen partial pressure, for example 1 kPa or
less, are stable in relation to oxidation as long as the platinum wire is
not contaminated. In conclusion, the oxygen in the sheath should be lower
than 1 kPa, and if used in a broad temperature region, preferably should
be conducted by reduction to the reduction region of 600° C. or
more for every measurement.

(Experiment 3)

[0063]Next, an experiment was conducted regarding stability based on
sufficient oxidation. First, after an experiment was conducted to
investigate the oxidation characteristics in the low temperature region
using three types of platinum resistance thermometers, Y002 (2 kPa), Y003
(2 kPa), and S4742 (8 kPa), and then reducing each at 650° C. for
approximately 15 hours, the temperature was maintained at 100° C.
and 150° C. for 3 to 4 days. The resistance values of the platinum
resistance thermometers were periodically measured at the triple point of
water. FIG. 8 indicates the results of measuring the resistance value
variations of the platinum resistance thermometers at 100° C., and
FIG. 9 indicates the results of measuring the resistance value variations
of the platinum resistance thermometers at 50° C. The resistance
values indicated gradual drift following the PtO2 curve in FIG. 1.
It may be inferred from these resistance value variations that several
layers of oxidized platinum were on the surface of the platinum wire.
However, the resistance values continually increased, and the oxidation
layers expanded.

[0064]According to the oxidation potential phase diagram, resistance value
drift of the platinum resistance thermometers caused by oxidation of
platinum cannot be avoided, but drift caused by oxidation is not that
rapid at room temperature. If the platinum resistance thermometer can be
controlled to a given oxidation state, then stability could probably be
guaranteed over a long period in a limited temperature use region. From
this perspective, the stability of sufficiently oxidized platinum wire
was measured. The results are indicated in FIG. 10. In this experiment,
the aforementioned 3 platinum resistance thermometers were maintained at
420° C. for approximately 15 hours, and afterwards were slowly
cooled at a rate of about -20° C./h in order to cause sufficient
oxidation of the platinum wire. Next, the resistance values at the triple
point of water were measured. Drift was observed for the first several
hours, but afterwards the resistance stabilized. The results of
Experiment 3 demonstrate that platinum wire that has been sufficiently
oxidized is comparatively stable, and if in the aforementioned PtO2
oxidized form in the temperature region of use, the platinum resistance
thermometer can be stably used even at a partial pressure of 1 kPa or
more.